How do I add an HDRI texture environment node?
In the world section of the properties panel choose Surface > Use Nodes.

Select Surface: Background.

As color choose  Environment texture.

And choose the HDR image you want to use.



Or you can use the Node editor. Enable Use Nodes and in the shader editor and set it to world to add an environment texture node and connect it to a background node and then to the surface of the world output.